The present invention refers to a filter according to the preamble of claim 1. The invention also refers to a fuel assembly.
The invention will be described in an application for nuclear plants for cleaning the cooling water, which flows through a nuclear reactor of a light water type, from debris and other particles. However, the invention is not limited to any particular positioning of the filter in the nuclear plant.
It is important to clean the cooling water in a nuclear plant. The purpose of the cooling water is to function as a cooling fluid and a moderator in the nuclear reactor of the nuclear plant. If debris or other particles are permitted to follow the cooling water in the core of the reactor, these may cause defects to the cladding of the fuel rods, which may result in such defects that nuclear fuel, i.e. uranium leaks out into the cooling water. At larger defect, the operation of the reactor has then to be interrupted and the failed fuel be replaced. Such a replacement is time-consuming and expensive. Debris and other particles may of course also cause defects to other components in a nuclear plant, for instance pumps.
Such debris may consist of metal chips, which are formed in connection with different repairs of components of the plant, metal wires or other foreign particles which have reached the plant from outside. Particularly difficult particles are such with an elongated shape, i.e. thin wires or chips which may have a length of down to 10 mm. Such particles tend to get attached to the fuel assembly at a higher level, for instance to spacers. The particles are vibrating in the cooling water stream and may wear the cladding of the fuel rods so that a hole arises. At the same time it is important not to filter away particles which are not considered to be dangerous since all matters which are caught by the filter increase the pressure drop across the filter. Such particles may for instance be blasting sand with the size of 1–2 mm and mineral particles, which may reach the cooling water in case of defects.
In order to solve this problem, it is known to provide some form of a filter in the lower part of the fuel assemblies, which include a number of fuel rods and which form the core of the reactor. The cooling water, which circulates through the reactor, passes through this lower part of the fuel assemblies. For instance, the bottom plate of the fuel assembly may be provided with a plurality of small holes through which the cooling water passes. Possible debris or other particles may thus be caught by such a filter. There are two important requirements on such a filter, on one hand it is to catch all particles which may cause defects in the reactor in an efficient manner and on the other hand it is to have a low flow resistance and pressure drop.
SE-B-465 192, U.S. Pat. No. 5,481,577 and U.S. Pat. No. 5,030,412 disclose different such filters for catching debris in the cooling water flowing through a nuclear reactor. SE-B-465 192 suggests holes in the bottom plate, which have different portions with centre lines that are displaced in relation to each other. U.S. Pat. No. 5,481,577 suggests a filter which consists of a number of sheets arranged beside each other and forming passages for the cooling water. The passages are relatively thin but have a large width, which makes it possible for elongated debris particles to pass. U.S. Pat. No. 5,030,412 discloses a filter, which includes a plane metal sheet, which has relatively elongated passages that let the cooling water through but stops possible particles. Upstream of the metal plate, parallel, substantially vertical sheets are provided at a small distance from each other. These parallel sheets have an intermediate curvature, which contributes to giving the particles a desired extension transversally to the cooling water flow before they reach the metal sheet.
DE-U-296 15 575 discloses another filter for a fuel assembly in a nuclear plant. The filter consists of a frame, in which a package of sheets are provided beside each other in such a way that cooling water passages are formed between the plates. The plates have a wave-shape, which either extends in a direction transversally to the flow direction or in the flow direction.